In the manufacture of polystyrene foam, it has been common to utilize blowing agents such as chlorofluorocarbons solely or together with alkanes, usually pentane or isopentane. Such a foam is usually made by extrusion. Due to the solubility of chlorofluorocarbons and certain alkanes in polystyrene, the extrusion melt viscosity is reduced and this permits sufficient cooling of the melt so that high load, indicated by amperage, will not be applied to the extruder drive motor.
Pentane and chlorofluorocarbons such as Freon 11 and 12, for example, are partially soluble in polystyrene and reduce the melt viscosity enabling the extruder adequately to cool and extrude the molten plastic material at lower loads and temperatures in order to generate a good smooth foam surface and appearance, small cell size control for opacity and other foam physical properties such as orientation, shrinkage characteristics and stiffness. Loss of residual blowing agent over a period of time, however, creates a shrinkage in the sheet material, an increase in thickness and an increase in softening point temperature as the residual blowing agent is reduced.
Where the foam is intended for use as a label on glass or plastic containers wherein the label is shrunk in place, the foam material is extruded and stretched in one direction more than the other. Preferably, the label material is coextruded with a plastic film. The gradual loss of residual blowing agent causes the label material to vary in shrinkage characteristics.
U.S. Pat. No. 4,424,287 is directed to a polymer foam process for making thermoformed articles wherein the blowing agent comprises at least an atmospheric gas and at least one volatile plasticizing blowing agent. As stated in this patent, early attempts to mix normally liquid hydrocarbons and normally gaseous blowing agents have not been successful and it is necessary that great care be exercised in production of a polymer foam with highly-volatile blowing agents such as carbon dioxide even in such a mixture. Accordingly, as set forth in this patent, it was thought necessary to utilize both the inert gas such as carbon dioxide and a volatile plasticizing organic blowing agent such as pentane and to thermoform the articles immediately after extrusion.
U.S. Pat. No. 4,436,679 discloses a method of making foam wherein water and natural gas are introduced into a melt.
U.S. Pat. No. 4,470,938 discloses a method of utilizing atmospheric gases for forming thermoformed articles wherein the process includes introducing, to a molten resin charge under pressure, a gas blowing agent from a supply gas source of comparatively higher pressure, said introducing being by means of discrete uniform volumetric charges of gas into said resin, wherein said discrete volumetric charges are introduced by means of a free piston-cylinder combination given motive power by said supply gas source. Although this patent mentions the use of a number of atmospheric gases, no claim was made as to any beneficial results as to the product and no commercial use has been made of such a process.
Among the objectives of the present invention are to provide a method of making polystyrene foam wherein the method utilizes atmospheric gases as blowing agents which are inherent in the atmosphere; wherein the blowing agent is substantially entirely released when the polystyrene foam is extruded; wherein the melt can be cooled to normal process levels and not create high loading on the extruder or shear heat from high polymer viscosity; wherein the amount of shrinkage can be controlled as may be required for the ultimate use of the foam; wherein the density and cell size can be controlled; and which utilizes conventional extrusion apparatus.
In accordance with the invention, the method of making polystyrene foam comprises mixing an atmospheric gas such as carbon dioxide, nitrogen or air and preferably a non-volatile blending agent, introducing the gas or the mixture to a molten thermoplastic polystyrene resin containing nucleating agents such as sodium bicarbonate and citric acid and extruding the thermoplastic in the form of a foam. The resultant foam has substantially no residual blowing agent, is dimensionally stable at ambient temperatures and when used with a non-volatile blending agent can be made with small amounts of blowing agent permitting melt cooling without high loading on the extruder. The foam may be coextruded with one or more layers of a thermoplastic film so that the resultant product is adapted for use for plates, cups, food containers or packaging and for labels for containers such as glass and plastic containers and for labels that are shrunk in place on the container.